Prior to this invention there were several known methods for the preparation of both pentachloro-2-propanone, hereinafter referred to as pentachloroacetone, and dichloroacetic acid, but none of these employed the novel process of this invention.
One of the known methods for preparing pentachloroacetone is the chlorination of isopropyl alcohol as disclosed in Hyym E. Buc., U.S. Pat. No. 1,391,757 (1927). Another is the chlorination of acetone in sunlight as pointed out in Huntress, Organic Chlorine Compounds, p. 812, John Wiley & Sons, Inc., New York, (1948). A commonly used method for the production of dichloroacetic acid is the chlorination of acetic acid. However, this method produces significant proportions of by-products including monochloroacetic and trichloroacetic acid which pose difficult separation problems for the isolation of the dichloroacetic acid; e.g., fractional distillation is difficult due to the closeness of the boiling points of these acids. Elaborate separation methods have been devised by others to alleviate this problem. None of these methods have proven completely satisfactory.
The process of this invention avoids the acid separation problem because it produces 95% pure dichloroacetic acid in high yields which is virtually free of the other chlorinated acetic acids.
A further advantage of this invention is that it readily allows utilization of di(chloroisopropyl) ether as a source material, such ether currently being a waste product of certain industrial processes.
The above advantages as well as others are now achievable because it has now been discovered that both pentachloroacetone and dichloroacetic can be prepared from certain isopropyl ethers. Pentachloroacetone is produced by chlorinating these ethers in the presence of water. Dichloroacetic acid is produced by chlorinating these ethers in the presence of water to produce pentachloroacetone, reacting this pentachloroacetone with an inorganic base-acting material to produce a salt of the dichloroacetic acid, and reacting this salt with a strong mineral acid.
More specifically the process of this invention can be described as follows.
Pentachloroacetone is produced by reacting chlorine in the presence of water with at least one of the isopropyl ethers having either the formula C.sub.3 H.sub.(7.sub.-x) Cl.sub.x --O--C.sub.3 H.sub.(7.sub.-y) Cl.sub.y or C.sub.3 H.sub.(7.sub.-x) Cl.sub.x --O--R or a mixture thereof, where x and y are integers independently selected from those integers from zero to five, inclusive, and R is selected from the group consisting of lower alkyl groups and chlorinated lower alkyl groups where lower alkyl is defined as an alkyl group containing from 1 to 6 carbon atoms. This chlorination step is carried out at a temperature of from about 25.degree. to about 300.degree. C and a pressure of from about 0.5 to about 10 atmospheres for a time sufficient to produce an optimum amount of pentachloroacetone, such time being from as low as about 2 to about 6 hours to as high as about 60 hours. The chlorine to ethers molar ratio for this chlorination step is not so critical as to require one specific ratio, but in order to prevent substantial underchlorination or overchlorination, both of which reduce the pentachloroacetone yield, a molar ratio should be used which produces acceptable yields. For ethers having the formula C.sub.3 H.sub.(7.sub.-x) Cl.sub.x --O--C.sub.3 H.sub.(7.sub.-y) Cl.sub.y, the theoretical number of moles of chlorine needed per mole of ether is given by the mathematical expression [(6-x) + (6-y)], where x and y have the same numerical value as they do in the formula for the ether. For those ethers having the general formula C.sub.3 H.sub.(7.sub.-x) Cl.sub.x --O--R, the theoretical number of moles of chlorine required per mole of ethers is given by the mathematical term (6-x), where x has the same numerical value as it does in this ether general formula. However, acceptable yields result when molar ratios other than the theoretical ratios are used. For example, di(chloroisopropyl) ether, C.sub.3 H.sub.6 Cl--O--C.sub.3 H.sub.6 Cl, produces acceptable yields when using a chlorine to ether molar ratio of from about 8:1 to about 20:1 while a molar ratio of from about 10:1 to about 14:1 is preferred. It may be noted that the theoretical ratio would be 10:1 for this particular ether, with x=1 and y=1.
The ethers represented by the general formula C.sub.3 H.sub.(7.sub.-x) Cl.sub.x --O--C.sub.3 H.sub.(7.sub.-y) Cl.sub.y, are acceptably chlorinated with a chlorine to ether mole ratio of from about 0.8[(6-x) + (6-y)]:1 to about 2[(6-x) + (6-y)]:1 with a preferred ratio of from about [(6-x) + (6-y)]:1 to about 1.4[(6-x) + (6-y)]:1, where x and y are integers independently selected from zero to five, but each having the same numerical value in the ratio as they do in the particular ether or mixtures of ethers being chlorinated.
For the ethers represented by the general formula C.sub.3 H.sub.(7.sub.-x) Cl.sub.x --O--R, acceptable chlorination of the isopropyl group occurs when the molar ratio of chlorine to ether is from about 0.8(6-x):1 to about 2(6-x):1 with a preferable ratio being about (6-x):1 to about 1.4(6-x):1 where x is an integer from zero to 5, but being the same in the particular ether or ethers chlorinated as in the ratio, and R is selected from lower alkyl or chlorinated lower alkyl groups. The molar ratios above do not include the extra chlorine that is required if R reacts with the chlorine. An allowance would have to be made for this extra chlorine, with the molar ratio values of chlorine to ether increased accordingly to compensate therefor.
There is sufficient time during this chlorination step to determine when the optimum or other desired amount of pentachloroacetone is attained by methods such as gas chromatographic analysis. Once the desired amount is determined along with the time it took to achieve this amount, subsequent runs can be made without the gas chromatography by merely using the predetermined reaction time. Overchlorination results in excess production of hexachloroacetone which immediately breaks down into chloroform and carbon dioxide since it is unstable under these reaction conditions. Underchlorination yields more tetra-, tri-, di-, and even some monochloroacetone depending upon the extent of the underchlorination. By chlorinating the isopropyl ethers until there is virtually no mono-, di-, tri-, nor tetrachloroacetone remaining the only chlorinated acetone present in substantial quantities is pentachloroacetone since hexachloroacetone is unstable under the reaction conditions.
Without the presence of the other chlorinated acetones, pentachloroacetone can be readily converted to dichloroacetic acid of high purity in good yields and with insignificant amounts of mono- or trichloroacetic acid being present.
If dichloroacetic acid is desired, it is prepared by a process which comprises using the identical process for the production of pentachloroacetone described above, taking the pentachloroacetone thus produced and reacting it with a fairly strong base-acting material such as sodium hydroxide to produce a salt of dichloroacetic acid, and then reacting this salt with a strong mineral acid such as HCl to produce dichloroacetic acid. The base-acting material is a member selected from a group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, and mixtures thereof.